The present invention relates to optical comb filters, and more particularly to an optical comb filter having a comb filter characteristic with attenuation points at a plurality of specific space frequencies and at frequencies which are odd-number multiples of the specific space frequencies.
In a color television camera employing a color resolution stripped filter of a known type, an output signal results from a chrominance signal modulating a signal having a space frequency value corresponding to a constantly repeated arrangement of fine color stripes of the color resolution filter positioned in the light path between the object being picked up and the photoelectric surface or photoconductive layer of the camera tube. Accordingly, interference occurs between the signal of the specific space frequency, determined by the color resolution striped filter, and the signal obtained from the object as a result, a beat disturbance appears in the reproduced picture.
Accordingly, with the aim of solving this problem, a known method uses an optical low-pass filter which depends on a device such as a lenticular lens or a phase grid plate. The light image of the color resolution striped filter is applied in a defocused state, in only the horizontal scanning line direction, to the photoconductive layer of the camera tube.
This previously known method encountered problems owing to such causes as variation in optical characteristics, variation in light due to adjustment of the camera lens aperture, the installation position, and a nonuniformity of the refractive index and transmission index. The striped pattern of the lenticular lens of the phase grid plate is projected onto the photoconductive layer in some instances. In such a case, an additional interference occurs between the striped pattern of the lenticular lens or the phase grid plate and the striped pattern of the color resolution striped filter. Beats are disadvantageously produced in the reproduced picture.
Another known method is intended to solve the above described problem and to overcome the difficulty of the above described method. A flat plate of a transparent anisotropic substance having a double refraction characteristic is used as an optical comb filter. This optical comb filter has a characteristic with attenuation points at the specific space frequency, according to an arrangement of a constant repetition of the fine color stripes of the color resolution striped filter. Attenuation also occurs at frequencies which are odd-number multiples of the space frequency. There are pass bands for DC component at frequencies which are even-number multiples of the space frequency. In this optical comb filter, there is an offset distance .DELTA.l between a normal light (normal light ray) and an abnormal light (abnormal light ray) appearing on the light exiting surface side of the filter as a result of light entering the incident surface side and undergoing double refraction. If the offset distance .DELTA.l becomes one half of the repetition period of the space frequency of the object, the signal component of this space frequency of the object is removed.
However, there is a difficulty accompanying this optical comb filter. If the incident light entering this filter has a specific polarization, a difference arises in the light quantities of the normal light and the abnormal light appearing on the light-exiting surface side of this filter. Then, satisfactory comb characteristics can no longer be exhibited. Furthermore, in the extreme case, only either the normal light or the abnormal light appears on the light-exiting surface of the optical comb filter. Then, the comb filter characteristics become completely lost. Furthermore, if a semitransparent mirror is provided on the front surface of an optical comb filter, or if the object has a uniform reflection surface, an incident light projected into the incidence surface of the filter becomes polarized light. Consequently, the normal light and the abnormal light exit, with a great difference in light quantity, from the light-exiting surface of the comb filter. In this case, the optical comb filter cannot function as one having desirable comb filter characteristics.
If a plurality of signals of a specific space frequency are generated by the color resolution striped filter, it is necessary to remove the components of the specific space frequency by means of an optical comb filter. However, an optical comb filter employing a flat plate of a transparent anisotropic substance has the above mentioned double refraction characteristic, with attenuation points at a specific space frequency and at odd-number multiples thereof. Accordingly, a plurality of signals having different space frequencies, without mutual relationship of odd-number multiples, cannot be removed.
Accordingly, for removing these space frequencies, the combination of a plurality of optical comb filter elements capable of separately removing respective space frequencies would appear to be feasible.
However, as described hereinafter in conjunction with the drawing, an optical comb filter having attenuation points in a plurality of space frequencies cannot be obtained by merely combining a plurality of optical comb filters. The only result of such a combination will be the production of an optical comb filter of a characteristic in which there are attenuation points at one frequency which is different from any of the removal frequencies and at frequencies which are odd-number multiples of that one frequency.